In the field of optical communications, semiconductor substrates on each of which a plurality of semiconductor optical elements are integrated have been widely used. On such a semiconductor substrate, a light waveguide which includes a core made of a semiconductor is formed by a process that is highly compatible with a process for forming semiconductor elements. Such a light waveguide is used to, for example, connect a semiconductor element to another semiconductor element.
On such a semiconductor substrate, a light combining/dividing element is formed in some cases. Note, here, that a light combining/dividing element indicates an element having (i) a function of combining rays of light, having been caused to enter a respective plurality of light waveguides, so that a ray of light is outputted from a single light waveguide and (ii) a function of dividing a ray of light, having been caused to enter the single light waveguide, so that rays of light are outputted from the respective plurality of light waveguides.
Each of Patent literatures 1 and 2 discloses a light combining/dividing element configured such that a single light waveguide (first waveguide) is connected to two light waveguides (second and third waveguides) via a multi-mode interference (MMI) section, that is, a 1×2 light combining/dividing element having an MMI section.
FIG. 1 of Patent Literature 2 illustrates a light combining/dividing element configured such that (1) a first waveguide (the second input/output waveguide section illustrated in FIG. 1) is connected to an MMI section (the first multi-mode waveguide section illustrated in FIG. 1) via a first tapered section (the second multi-mode waveguide section and the fourth input/output waveguide section each illustrated in FIG. 1) and (2) a second waveguide and a third waveguide (the first input/output waveguide section illustrated in FIG. 1) are each connected to the MMI section via a second tapered section (the third input/output waveguide section illustrated in FIG. 1).
According to the above configuration, a difference in effective refractive index between the first waveguide and the MMI section is reduced by an effective refractive index of the first tapered section. It is therefore possible to suppress light loss resulting from light reflection caused by connecting the first waveguide to the MMI section.
Meanwhile, Non-Patent Literature 1 discloses a light crossing element configured such that a first waveguide and a second waveguide are crossed on an identical plane surface. As illustrated in FIG. 2 of Non-Patent Literature 1, the light crossing element includes the first waveguide and the second waveguide which are provided so as to be perpendicular to each other, and an expanded region is formed in a vicinity of an crossing at which the first waveguide and the second waveguide are crossed.
A shape of the expanded region has fourfold symmetry, when the expanded region is viewed from above. The expanded region is made up of (i) a first ellipse whose long axis matches a central axis of the first waveguide and (ii) a second ellipse whose long axis matches a central axis of the second waveguide. The expanded region is arranged so as to be thinner than each of the first waveguide and the second waveguide.
According to the above configuration, it is possible to reduce crosstalk between (i) light guided through the first waveguide and (ii) light guided through the second waveguide, while suppressing light loss resulting from light scattering which occurs at the crossing.
According the light combining/dividing element disclosed in Patent Literature 2, it is considered that a light combining function and a light dividing function are realized with active use of interference that is an interaction between rays of light which interaction occurs in the MMI section.
Meanwhile, according to the light crossing element disclosed in Non-Patent Literature 1, it is considered that an interaction, between (i) light guided through the first waveguide and (ii) light guided through the second waveguide, causes crosstalk between these rays of light. That is, according to the light crossing element, it is considered that, by suppressing such an interaction as much as possible, crossing of rays of light is realized while crosstalk between the rays of light is being reduced.
As such, the light combining/dividing element and the light crossing element are inventions based on respective opposite ideas about an interaction between rays of light.